U.S. patent application number 15/697505 was filed with the patent office on 2019-03-07 for hermetically sealed molecular spectroscopy cell with buried ground plane.
The applicant listed for this patent is TEXAS INSTRUMENTS INCORPORATED. Invention is credited to Benjamin Stassen COOK, Adam Joseph FRUEHLING, Juan Alejandro HERBSOMMER, Simon Joshua JACOBS.
Application Number | 20190074233 15/697505 |
Document ID | / |
Family ID | 65518740 |
Filed Date | 2019-03-07 |
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United States Patent
Application |
20190074233 |
Kind Code |
A1 |
FRUEHLING; Adam Joseph ; et
al. |
March 7, 2019 |
HERMETICALLY SEALED MOLECULAR SPECTROSCOPY CELL WITH BURIED GROUND
PLANE
Abstract
A method for forming a sealed cavity includes bonding a
non-conductive structure to a first substrate to form a
non-conductive aperture into the first substrate. On a surface of
the non-conductive structure opposite the first substrate, the
method includes depositing a first metal layer. The method further
includes patterning a first iris in the first metal layer,
depositing a first dielectric layer on a surface of the first metal
layer opposite the non-conductive structure, and patterning an
antenna on a surface of the first dielectric layer opposite the
first metal layer. The method also includes creating a cavity in
the first substrate, depositing a second metal layer on a surface
of the cavity, patterning a second iris in the second metal layer,
and bonding a second substrate to a surface of the first substrate
opposite the non-conductive structure to thereby seal the
cavity.
Inventors: |
FRUEHLING; Adam Joseph;
(Garland, TX) ; HERBSOMMER; Juan Alejandro;
(Allen, TX) ; JACOBS; Simon Joshua; (Lucas,
TX) ; COOK; Benjamin Stassen; (Addison, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TEXAS INSTRUMENTS INCORPORATED |
Dallas |
TX |
US |
|
|
Family ID: |
65518740 |
Appl. No.: |
15/697505 |
Filed: |
September 7, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 23/08 20130101;
B81B 2203/0315 20130101; B81C 1/00333 20130101; B81C 2203/0136
20130101; H01L 21/76898 20130101; G04F 5/14 20130101; B81C
2203/0145 20130101; B81C 1/00047 20130101 |
International
Class: |
H01L 23/08 20060101
H01L023/08; B81C 1/00 20060101 B81C001/00; H01L 21/768 20060101
H01L021/768 |
Claims
1. A method for forming a sealed cavity, comprising: bonding a
non-conductive structure to a first substrate to form a
non-conductive aperture into the first substrate; on a surface of
the non-conductive structure opposite the first substrate,
depositing a first metal layer; patterning a first iris in the
first metal layer; depositing a first dielectric layer on a surface
of the first metal layer opposite the non-conductive structure;
patterning an antenna on a surface of the first dielectric layer
opposite the first metal layer; creating a cavity in the first
substrate; depositing a second metal layer on a surface of the
cavity; patterning a second iris in the second metal layer; and
bonding a second substrate to a surface of the first substrate
opposite the non-conductive structure to thereby seal the
cavity.
2. The method of claim 1, further comprising depositing a second
dielectric layer on a surface of the first dielectric opposite the
first metal layer.
3. The method of claim 2, further comprising depositing and
patterning an electronic bandgap structure on a surface of the
second dielectric layer opposite the first dielectric layer.
4. The method of claim 1, wherein creating the cavity comprises at
least one of wet etching, reactive-ion etching, deep reactive-ion
etching, or isotropic etching the cavity.
5. The method of claim 4, wherein the wet etching uses at least one
of potassium hydroxide (KOH) and tetramethylammonium hydroxide
(TMAH) as a wet etchant.
6. The method of claim 1, wherein bonding the second substrate to
the first substrate comprises at least one of eutectic bonding,
anodic bonding, fusion bonding, transition liquid phase (TLP)
bonding, cofiring, or glass-to-metal hermetic sealing.
7. The method of claim 1, wherein patterning the second iris
comprises patterning the iris in the second metal layer so as to
align with the first iris along a vertical axis through the
non-conductive structure.
8. The method of claim 1, wherein bonding the non-conductive
structure to the first substrate comprises bonding a glass sheet to
the first substrate.
9. The method of claim 1, wherein bonding the non-conductive
structure to the first substrate comprises bonding the
non-conductive structure to a semiconductor wafer.
10. The method of claim 1, wherein bonding the non-conductive
structure to the first substrate comprises bonding a glass sheet to
a semiconductor wafer.
11. A device, comprising: a non-conductive structure; a first
substrate bonded to the non-conductive structure, wherein the first
substrate includes a cavity that extends from the non-conductive
structure to a surface of the first substrate opposite the
non-conductive structure; a second substrate bonded to the first
substrate to seal the cavity; a plurality of dipolar molecules
disposed within the cavity; a first metal layer on a surface of the
non-conductive structure opposite the first substrate; a first
dielectric layer on a surface of the first metal layer opposite the
non-conductive structure; and a first antenna patterned on a
surface of the first dielectric layer opposite the first metal
layer.
12. The device of claim 11, further comprising a second dielectric
layer on a surface of the first dielectric layer opposite the first
metal layer.
13. The device of claim 12, further comprising an electronic
bandgap structure on a surface of the second dielectric layer
opposite the first dielectric layer.
14. The device of claim 11, wherein the dipolar molecules are water
molecules and the cavity has a pressure of less than 0.015
mbars.
15. The device of claim 11, wherein the first metal layer includes
an aperture vertically aligned with the cavity.
16. The device of claim 11, further comprising an amplifier, a
filter, a signal generator, and a second antenna patterned on the
first dielectric layer, wherein: the signal generator is coupled to
the first antenna and is configured to generate a transmit signal
to the first antenna; the amplifier is coupled to the second
antenna and is configured to generate an error signal based on a
receive signal from the second antenna and the transmit signal; and
the filter is coupled to the amplifier and the signal generate, and
is configured to generate a control output signal, based on the
error signal, to adjust a frequency of the transmit signal
generated by the signal generator.
17. A device, comprising: a glass sheet; a first semiconductor
substrate bonded to the glass sheet, wherein the first
semiconductor substrate includes a cavity that extends from the
glass sheet to a surface of the first semiconductor substrate
opposite the glass sheet; a second semiconductor substrate bonded
to the first semiconductor substrate to seal the cavity; a
plurality of dipolar molecules disposed within the cavity; a first
metal layer on a surface of the glass sheet opposite the first
semiconductor substrate; a first dielectric layer on a surface of
the first metal layer opposite the glass sheet; first and second
antennas patterned on a surface of the first dielectric layer
opposite the first metal layer; and a transceiver electrically
coupled to the first and second antennas and configured to inject a
transmit signal into the cavity through the first antenna, generate
an error signal based on the transmit signal and a receive signal
from the second antenna, and dynamically adjust a frequency of the
transmit signal based on the error signal.
18. The device of claim 17, further comprising: a second dielectric
layer on a surface of the first dielectric layer opposite the first
metal layer; and an electronic bandgap structure on a surface of
the second dielectric layer opposite the first dielectric
layer.
19. The device of claim 17, the dipolar molecules are water
molecules and the cavity has a pressure of less than 0.15
mbars.
20. The device of claim 14, wherein the transceiver includes: a
signal generator coupled to the first antenna and configured to
generate the transmit signal; an amplifier coupled to the second
antenna and configured to generate the error signal; and a loop
filter coupled to the amplifier and the signal generator, wherein
the loop filter is configured to, based on the error signal,
generate a control output signal to the signal generator.
Description
BACKGROUND
[0001] Various applications may include a sealed chamber formed in
a semiconductor structure. Although there are numerous applications
of sealed chamber, in one particular application, a chip-scale
atomic dock may include a selected vapor at a low pressure in a
sealed chamber. Forming such structures can be a challenge.
SUMMARY
[0002] Methods for forming a hermetically sealed cavity, and the
resulting cavity, are described herein. In one embodiment, a method
for forming a sealed cavity includes bonding a non-conductive
structure to a first substrate to form a non-conductive aperture
into the first substrate. On a surface of the non-conductive
structure opposite the first substrate, the method includes
depositing a first metal layer. The method further includes
patterning a first iris in the first metal layer, depositing a
first dielectric layer on a surface of the first metal layer
opposite the non-conductive structure, and patterning an antenna on
a surface of the first dielectric layer opposite the first metal
layer. The method also includes the deposition or bonding of
further dielectric and metal layers and their subsequent patterning
on the topmost surface to improve the radio frequency (RF)
performance of antenna, transmission line structures, and
electromagnetic bandgap structures. The method also includes
creating a cavity in the first substrate, depositing a second metal
layer on a surface of the cavity, patterning a second iris in the
second metal layer, and bonding a second substrate to a surface of
the first substrate opposite the non-conductive structure to
thereby seal the cavity.
[0003] Another embodiment includes a device that includes a
non-conductive structure and a first substrate bonded to the
non-conductive structure. The first substrate includes a cavity
that extends from the non-conductive structure to a surface of the
first substrate opposite the non-conductive structure. The device
further includes a second substrate bonded to the first substrate
to seal the cavity, a plurality of dipolar molecules disposed
within the cavity, a first metal layer on a surface of the
non-conductive structure opposite the first substrate, a first
dielectric layer on a surface of the first metal layer opposite the
non-conductive structure, and a first antenna patterned on a
surface of the first dielectric layer opposite the first metal
layer.
[0004] In yet another embodiment, a device includes a glass sheet
and a first semiconductor substrate bonded to the glass sheet. The
first semiconductor substrate includes a cavity that extends from
the glass sheet to a surface of the first semiconductor substrate
opposite the glass sheet. The device also includes a second
semiconductor substrate bonded to the first semiconductor substrate
to seal the cavity, a plurality of dipolar molecules disposed
within the cavity, a first metal layer on a surface of the glass
sheet opposite the first semiconductor substrate, a first
dielectric layer on a surface of the first metal layer opposite the
glass sheet, and first and second antennas patterned on a surface
of the first dielectric layer opposite the first metal layer. The
device further includes a transceiver electrically coupled to the
first and second antennas and configured to inject a transmit
signal into the cavity through the first antenna, generate an error
signal based on the transmit signal and a receive signal from the
second antenna, and dynamically adjust a frequency of the transmit
signal based on the error signal
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] For a detailed description of various examples, reference
will now be made to the accompanying drawings in which:
[0006] FIGS. 1A-1I illustrate a sequence of processing operations
in one embodiment to form a hermetically sealed cavity;
[0007] FIG. 2 illustrates a method flow chart to form a
hermetically sealed cavity in accordance with various
embodiments;
[0008] FIGS. 3A-3E illustrate a sequence of processing operations
in accordance with another embodiment to form a hermetically sealed
cavity;
[0009] FIG. 4 illustrates another method flow chart to form a
hermetically sealed cavity in accordance with various
embodiments
[0010] FIG. 5 shows a cross-sectional view of the hermetically
sealed cavity of various embodiments; and
[0011] FIG. 6 shows a block diagram for a clock generator in
accordance with various embodiments.
DETAILED DESCRIPTION
[0012] Certain terms are used throughout the following description
and claims to refer to particular system components. As one skilled
in the art will appreciate, different parties may refer to a
component by different names. This document does not intend to
distinguish between components that differ in name but not
function. In the following discussion and in the claims, the terms
"including" and "comprising" are used in an open-ended fashion, and
thus should be interpreted to mean "including, but not limited to .
. . " Also, the term "couple" or "couples" is intended to mean
either an indirect or direct wired or wireless connection. Thus, if
a first device couples to a second device, that connection may be
through a direct connection or through an indirect connection via
other devices and connections. The recitation "based on" is
intended to mean "based at least in part on." Therefore, if X is
based on Y, X may be a function of Y and any number of other
factors.
[0013] The disclosed embodiments of the present disclosure include
techniques to fabricate a hermetically sealed cavity in a
substrate. A structure containing a substrate with the cavity may
be used in numerous applications. One illustrative use is as a
millimeter wave chip scale atomic clock. The cavity contains a
plurality of dipolar molecules (e.g., water molecules) at a
relatively low pressure. For some embodiments, the pressure is
approximately 0.1 mbarr for water molecules. If argon molecules
were used, the pressure may be several atmospheres. The
hermetically sealed cavity contains selected dipolar molecules at a
pressure chosen to optimize the amplitude of a signal absorption
peak of the molecules detected at an output of the cavity. An
electromagnetic signal is injected through aperture into the
cavity. Through closed-loop control, the frequency of the signal is
dynamically adjusted to match the frequency corresponding to the
absorption peak of the molecules in the cavity. The frequency
produced by quantum rotation of the selected dipolar molecules may
be unaffected by circuit aging and may not vary with temperature or
other environmental factors.
[0014] While a variety of materials and manufacturing operations
can be employed, one illustrative method includes bonding a
non-conductive structure (e.g., glass) to a first substrate (e.g.,
a semiconductor wafer) to form a non-conductive aperture into the
first substrate. On a surface of the non-conductive structure
opposite the first substrate, the method includes depositing a
first metal layer. A first iris is patterned on the first metal
layer. The method further includes depositing a first dielectric
layer on a surface of the first metal layer opposite the
non-conductive structure, patterning an antenna on a surface of the
first dielectric layer opposite the first metal layer, and creating
a cavity in the first substrate. Further, the method includes
depositing a second metal layer on a surface of the cavity,
patterning a second iris in the second metal layer, and bonding a
second substrate (e.g., another semiconductor wafer) to a surface
of the first substrate opposite the non-conductive structure to
thereby seal the cavity.
[0015] FIGS. 1A-1I illustrate a sequence of process steps to
fabricate a hermetically sealed cavity in accordance with an
embodiment. At FIG. 1A, a non-conductive structure 102 is bonded to
a substrate 120 to form a non-conductive aperture for the substrate
120. In some embodiments, the non-conductive structure 102
comprises glass, but the non-conductive structure 102 can be other
than glass in other embodiments such as ceramic and silicon. The
substrate 120 is a semiconductor substrate (e.g., silicon) in some
embodiments, but can be other than a semiconductor substrate in
other embodiments, such as a ceramic material or a metallic cavity.
In one example, a glass sheet (e.g., 130 micrometers thick) is
bonded to a surface of a semiconductor wafer. The process to bond
the non-conductive structure 102 to the substrate 120 may comprise
an anodic, fusion, eutectic solder, transition liquid phase (TLP),
cofiring, or other suitable bonding processes.
[0016] FIG. 1B illustrates that a metal layer 104 has been
deposited on a surface of the non-conductive structure 102 opposite
the substrate 120. The metal layer 104 include any suitable metal
material such as copper, gold, etc. The metal layer 104 is a ground
plane. An iris 107 has been patterned in the metal layer 104. The
iris is patterned by removing a portion of the metal layer 104
(e.g., by wet etch, dry etch, liftoff, or other suitable processes)
FIG. 1B also illustrates a first dielectric layer 106 deposited on
a surface of the metal layer 104 opposite the non-conductive
structure 102. The first dielectric layer 106 comprises any
suitable type of dielectric material such as polymide, nitride, or
oxide. An antenna 108 has been patterned on a surface of the first
dielectric layer 106 opposite the metal layer 104. The antenna 108
comprises a conductive material such as copper or gold and an
electrical signal can be provided to the antenna or received from
the antenna. In some embodiments, one antenna is used to both
transmit and receive signals. In other embodiments, a pair of
antennas is patterned on the dielectric layer, and one antenna is
used to inject a signal into the cavity and another antenna is used
to receive a signal from the cavity. In such examples, the antennas
are located at or near opposite ends of the cavity.
[0017] FIG. 1C illustrates that a second dielectric layer 110 has
been deposited on a surface of the first dielectric layer 106
opposite the metal layer 104. The second dielectric layer 110
covers some or all of the antenna 108 as well. The second
dielectric layer 110 comprises the same type of material as the
first dielectric layer, or may comprise a different type of
dielectric material as the first dielectric layer. An electronic
bandgap (EBG) structure 112 also is deposited and patterned on a
surface of the second dielectric layer 110 opposite the first
dielectric layer 106. In operation, the EBG structure 112
attenuates electromagnetic wave coupling along the outer surface
111 of the second dielectric layer 110 between the antennas. The
EBG structure 112 helps to force the energy from the input signal
received through an antenna (e.g., antenna 108) into the cavity
125. Layers 110 and 112 are used to enhance the performance of the
feed structure patterned on antenna 108. Layers 110 and 112 also
serve as a reflector/ground plane to improve antenna efficiency for
the transmission or reception of the signal passed through the
cavity.
[0018] In the example of FIG. 1D, a cavity 125 has been created in
the substrate 120. The cavity 125 may be wet etched into the
substrate 120 using a suitable wet etchant such as potassium
hydroxide (KOH) or tetramethylammonium hydroxide (TMAH).
Alternatively, the cavity 125 can be formed by way of reactive-ion
etching (RIE), deep reactive-ion etching (DRIE), or isotropic
etching. The cavity 125 is etched from the surface 126 of the
substrate 120 opposite the non-conductive structure 102 to the
non-conductive structure 102 thereby exposing a portion of the
non-conductive structure 102. FIG. 1E illustrates that another
metal layer 130 has been deposited on a surface of the substrate
120 opposite the non-conductive structure 102. The metal layer 130
also is deposited in the cavity 125 as shown.
[0019] FIG. 1F illustrates that another iris 140 is created in the
second metal layer 130 within the cavity 125. The iris 140 is
patterned in the second metal layer 130 so that irises 107 and 140
align along a vertical axis 138 through the non-conductive
structure.
[0020] FIG. 1G shows another substrate 150 on which a metal layer
152 has been deposited. The substrate 150 comprises the same or
different material as substrate 120. In one example, the substrate
150 comprises a semiconductor substrate such as a silicon wafer,
but can be other than a semiconductor material in other examples.
FIG. 1H illustrates that bonding structures 145 are deposited and
patterned on either or both of the substrates 120 and 150. In one
example, the bonding structures comprise a gold, aluminum, silicon
or other types of material that form an alloy when heated to a
suitable temperature. FIG. 1I illustrates the resulting device
which includes a hermetically sealed cavity. Dipolar molecules are
trapped inside the cavity 125. The hermetically sealed cavity 125
contains dipolar molecules (e.g., water molecules) at an internal
pressure of less than, in one example, 0.15 mbars.
[0021] The flow chart of FIG. 2 illustrates a method in accordance
with an example. The operations may be performed in the order
shown, or in a different order. Further, the operations may be
performed sequentially, or two or more of the operations may be
performed concurrently.
[0022] At 202, the method includes bonding a non-conductive
structure (e.g., glass) to a first substrate (e.g., a semiconductor
wafer) to form a non-conductive aperture into the first substrate.
On a surface of the non-conductive structure opposite the first
substrate, the method further includes at 204 depositing a first
metal layer (e.g., metal layer 104). At 206, the method includes
patterning a first iris in the first metal layer. Patterning the
iris may comprise etching the first metal layer. At 208, the method
includes depositing a first dielectric layer (e.g., dielectric
layer 106) on a surface of the first metal layer opposite the
non-conductive structure (e.g., non-conductive structure 102) and,
at 210, patterning a transmission line and an antenna (e.g.,
antenna 108) on a surface of the first dielectric layer opposite
the first metal layer.
[0023] At 212, the method includes depositing a second dielectric
layer (e.g., dielectric layer 110) on a surface of the first
dielectric layer (e.g., dielectric layer 106) opposite the first
metal layer (e.g., metal layer 104). An EBG structure (e.g., EBG
structure 112) is deposited and patterned on a surface of the
second dielectric layer opposite the first dielectric layer
(operation 214). A cavity (e.g., cavity 125) is then created at 216
in the first substrate. The cavity may be created by wet etching
the first substrate.
[0024] At 218, the method includes depositing a second metal layer
(e.g., metal layer 130) on a surface of the cavity. The second
metal layer also is deposited on the first substrate outside the
cavity itself such as is shown in the example of FIG. 1E. At 220, a
second iris (e.g., iris 140) is patterned in the second metal layer
such as is illustrated in FIG. 1F, and, at 222, a second substrate
(e.g., substrate 150) is bonded to the first substrate on a surface
of the first substrate opposite the non-conductive structure to
thereby seal the cavity.
[0025] FIGS. 3A-3E illustrate a sequence of process steps to
fabricate a hermetically sealed cavity in accordance with another
embodiment. At FIG. 3A, a non-conductive structure 302 is bonded to
a substrate 320 to form a non-conductive aperture for the substrate
320. In some embodiments, the non-conductive structure 302
comprises glass, but the non-conductive structure 302 can be other
than glass in other embodiments such as ceramic, silicon, or
oxide/nitride. The substrate 320 is a semiconductor substrate
(e.g., silicon) in some embodiments, but can be other than a
semiconductor substrate in other embodiments, such as a ceramic
material or metallic cavity. In one example, a glass sheet (e.g.,
130 micrometers thick) is bonded to a surface of a semiconductor
wafer. The process to bond the non-conductive structure 302 to the
substrate 320 may comprise an anodic, fusion, eutectic solder,
transition liquid phase (TLP), cofiring, or other suitable bonding
processes.
[0026] FIG. 3B illustrates that a metal layer 304 has been
deposited on a surface of the non-conductive structure 302 opposite
the substrate 320. The metal layer 304 includes any suitable metal
material such as copper, gold, etc. The metal layer 304 is a ground
plane. An iris 307 has been patterned in the metal layer 304. FIG.
3B also illustrates a first dielectric layer 306 deposited on a
surface of the metal layer 304 opposite the non-conductive
structure 302. The first dielectric layer 306 comprises any
suitable type of dielectric material such as polymide, nitride, or
oxide. An antenna 308 has been patterned on a surface of the first
dielectric layer 306 opposite the metal layer 304. The antenna 308
comprises a conductive material such as copper or gold and an
electrical signal can be provided to the antenna or received from
the antenna. In some embodiments, one antenna is used to both
transmit and receive signals. In other embodiments, a pair of
antennas is patterned on the dielectric layer, and one antenna is
used to inject a signal into the cavity and another antenna is used
to receive a signal from the cavity. In such examples, the antennas
are located at or near opposite ends of the cavity.
[0027] In FIG. 3C, a cavity 325 has been created in the substrate
320. The cavity 325 may be wet etched into the substrate 120 using
a suitable wet etchant such as potassium hydroxide (KOH) or
tetramethylammonium hydroxide (TMAH). The cavity 325 is etched from
the surface 326 of the substrate 320 opposite the non-conductive
structure 302 to the non-conductive structure 302 thereby exposing
a portion of the non-conductive structure 302. FIG. 3D illustrates
that another metal layer 330 has been deposited on a surface of the
substrate 320 opposite the non-conductive structure 302. The metal
layer 330 also is deposited in the cavity 325 as shown. An iris 340
is etched in the metal layer 330 as explained above with regard to
FIG. 1F. FIG. 3E illustrates another substrate 350 on which a metal
layer 352 has been deposited. The substrate 350 comprises the same
or different material as substrate 320. In one example, the
substrate 350 comprises a semiconductor substrate such as a silicon
wafer, but can be other than a semiconductor material in other
examples. Bonding structures 345 have been deposited and patterned
on either or both of the substrates 320 and 350. In one example,
the bonding structures comprise a gold, aluminum, silicon or other
types of material that form an alloy when heated to a suitable
temperature. By way of the bonding structures 345, the substrates
320 and 350 are bonded together.
[0028] The flow chart of FIG. 4 illustrates a method in accordance
with various examples. The operations may be performed in the order
shown, or in a different order. Further, the operations may be
performed sequentially, or two or more of the operations may be
performed concurrently. Some of the operations are the same or
similar as in the flow chart of FIG. 2.
[0029] At 402, the method includes bonding a non-conductive
structure (e.g., glass) to a first substrate (e.g., a semiconductor
wafer) to form a non-conductive aperture into the first substrate.
On a surface of the non-conductive structure opposite the first
substrate, the method further includes at 404 depositing a first
metal layer (e.g., metal layer 304). At 406, the method includes
patterning a first iris in the first metal layer. Patterning the
iris comprises etching the first metal layer. At 408, the method
includes depositing a first dielectric layer on a surface of the
first metal layer opposite the non-conductive structure and, at
410, patterning a transmission line and an antenna on a surface of
the first dielectric layer opposite the first metal layer. A cavity
is created at 412 in the first substrate. At 414, the method
includes depositing a second metal layer on a surface of the
cavity. At 416 and 418, the method further includes patterning a
second iris in the second metal layer and bonding a second
substrate to a surface of the first substrate opposite the
non-conductive structure to thereby seal the cavity.
[0030] FIG. 5 shows a cross-sectional view of a structure in
accordance with the disclosed embodiments. The structure comprises
a millimeter wave chip scale atomic clock. Substrate 120 is shown
bonded to substrate 150 with a hermetically sealed cavity 125
formed in the substrate 120 and sealed at least in part by
substrate 150. The non-conductive structure (e.g., glass) 102 is
shown bonded to the substrate 120 and irises 107 and 140 permit
electromagnetic energy to pass through the non-conductive structure
102 from the antenna 108 in the cavity 125. An example of the EBG
structure 112 also is shown in FIG. 5. Layer 104 provides a common
ground plane for all RF structures external to the cavity 125. In
addition, it limits propagation of waves travelling in either
layers 120 or 102. Layers 110 and 112 form transmission structures,
such as a stripline, for feeding RF signals to the antenna 108. In
the proximity of the antenna 108, it can also serve as a ground
reflector or defected ground structure. The dimensions of the
waveguide, antenna, EBG, and size and positioning of the iris 140
are all design considerations based on the chosen molecular species
inside the cavity and the wavelength of the interrogation waveform
within the cavity. The required bandwidth of the structure depends
upon the fabrication tolerances achievable in manufacturing.
[0031] FIG. 6 shows a block diagram for a clock generator 500
accordance with various embodiments. The clock generator 500 is a
millimeter wave atomic clock that generates a reference frequency
based on the frequency of quantum rotation of selected dipolar
molecules contained in a hermetically sealed cavity 102 formed in
semiconductor material. The reference frequency produced by quantum
rotation of the selected dipolar molecules is unaffected by circuit
aging and does not vary with temperature or other environmental
factors.
[0032] The clock generator 500 of FIG. 6 includes a vapor cell 505
formed in this example from substrates as described above. The cell
505 includes a cavity 508 with a sealed interior enclosing a
dipolar molecule material gas, for example, water (H.sub.2O) or any
other dipolar molecule gas at a relatively low gas pressure inside
the cavity 125. Non-limiting examples of suitable electrical
dipolar material gases include water, acetonitrile (CH.sub.3CN) and
hydrogen cyanide (HCN). As shown in FIG. 6, the clock generator 500
further includes a transceiver 600 with a transmit output 633 for
providing an electrical transmit signal (TX) to the vapor cell 505,
as well as a receiver input 638 for receiving an electrical input
signal (RX) from the vapor cell 525. The rotational transition
vapor cell 525 does not require optical interrogation, and instead
operates through electromagnetic interrogation via the transmit and
receive signals (TX, RX) provided by the transceiver 600.
[0033] The sealed cavity 508 includes a conductive interior cavity
surface, as well as first and second non-conductive apertures 515
and 517 formed in the interior cavity surface for providing an
electromagnetic field entrance and an electromagnetic field exit,
respectively. In one example, the apertures 515, 517 magnetically
couple into the TE10 mode of the cavity 508. In other examples, the
apertures 515, 517 excite higher order modes. First and second
conductive coupling structures 520 and 525 are formed on an outer
surface of the vapor cell 505 proximate the first and second
non-conductive aperture 515 and 517, respectively. The coupling
structures 520, 525 may be the antennas described above and may
comprise a conductive strip formed on a surface of one of the
substrates forming the cell 505. Each coupling structure 520, 525
overlies and crosses over the corresponding non-conductive aperture
515, 517 for providing an electromagnetic interface to couple a
magnetic field in to (based on the transmit signal TX from the
transceiver output 633) the cavity 508 or from the cavity to the
transceiver RX input 638 The proximate location of the conductive
coupling structures 520, 525 and the corresponding non-conductive
apertures 515, 525 advantageously provides electromagnetically
transmissive paths through the second or upper substrate 106, which
can be any electromagnetically transmissive material.
[0034] The transceiver circuit 600 in certain implementations is
implemented on or in an integrated circuit (not shown), to which
the vapor cell 505 is electrically coupled for transmission of the
TX signal via the output 633 and for receipt of the RX signal via
the input 638. The transceiver 600 is operable when powered for
providing an alternating electrical output signal TX to the first
conductive coupling structure 520 for coupling an electromagnetic
field to the interior of the cavity 508, as well as for receiving
the alternating electrical input signal RX from the second
conductive coupling structure 525 representing the electromagnetic
field received from the cavity 508. The transceiver circuit 600 is
operable for selectively adjusting the frequency of the electrical
output signal TX in order to reduce the electrical input signal RX
by interrogation to operate the clock generator 500 at a frequency
which substantially maximizes the molecular absorption through
rotational motor state transitions, and for providing a reference
clock signal REF_CLK at the frequency of the TX output signal.
[0035] In certain examples, the transceiver 600 includes a signal
generator 602 with an output 633 electrically coupled with the
first conductive coupling structure 520 for providing the
alternating electrical output signal TX, and for providing the
reference clock signal REF_CLK at the corresponding transmit output
frequency. The transceiver 600 also includes a lock-in amplifier
circuit 606 with an input 638 coupled from the second conductive
coupling structure 525 for receiving the RX signal. The lock-in
amplifier operates to provide an error signal ERR representing a
difference between the RX signal and the electrical output signal
TX. In one example, the lock-in amplifier 606 provides the error
signal ERR as an in-phase output, and the error signal ERR is used
as an input by a loop filter 604 to provide a control output signal
(CO) to the signal generator 602 for selectively adjusting the TX
output signal frequency to maintain this frequency at a peak
absorption frequency of the dipolar molecular gas inside the sealed
interior of the cavity 508. In some examples, the RF power of the
TX and RX loop is controlled so as to avoid or mitigate stark shift
affects.
[0036] The electromagnetic coupling via the non-conductive
apertures 520, 525 and corresponding conductive coupling structures
515, 517 facilitates electromagnetic interrogation of the dipolar
gas within the cell cavity 508. In one non-limiting form of
operation, the clock generator 500 operates with the signal
generator 602 transmitting alternating current (AC) TX signals at
full transmission power at various frequencies within a defined
band around a suspected quantum absorption frequency at which the
transmission efficiency of the vapor cell 505 is minimal
(absorption is maximal). For example, the quantum absorption
frequency associated with the dipolar water molecule is 183.31 GHz.
When the system operates at the quantum frequency, a null or minima
is detected at the receiver via the lock-in amplifier 606, which
provides the error signal ERR to the loop filter 604 for regulation
of the TX output signal frequency via the control output CO signal
provided to the signal generator 602. The rotational quantum
frequency of the dipolar molecule gas in the vapor cell cavity 508
is generally stable with respect to time (does not degrade or drift
over time), and is largely independent of temperature and a number
of other variables.
[0037] In one embodiment, the signal generator 602 initially sweeps
the transmission output frequency through a band known to include
the quantum frequency of the cell 505 (e.g., transitioning upward
from an initial frequency below the suspected quantum frequency, or
initially transitioning downward from an initial frequency above
the suspected quantum frequency, or other suitable sweeping
technique or approach). The transceiver 600 monitors the received
energy via the input 638 coupled with (e.g., electrically connected
to) the second conductive coupling structure 525 in order to
identify the transmission frequency associated with peak absorption
by the gas in the cell cavity 508 (e.g., minimal reception at the
receiver). Once the quantum absorption frequency is identified, the
loop filter 604 moves the source signal generator transmission
frequency close to that absorption frequency (e.g., 183.31 GHz),
and modulates the signal at a very low frequency to regulate
operation around the null or minima in the transmission efficiency
representing the ratio of the received energy to the transmitted
energy. The loop filter 604 provides negative feedback in a closed
loop operation to maintain the signal generator 602 operating at a
TX frequency corresponding to the quantum frequency of the cavity
dipolar molecule gas.
[0038] In steady state operation, the lock-in amplifier 606 and the
loop filter 604 maintain the transmitter frequency at the peak
absorption frequency of the cell gas. In one non-limiting example,
the loop filter 604 provides proportional-integral-derivative (PID)
control using a derivative of the frequency error as a control
factor for lock-in detection and closed loop regulation. At the
bottom of the null in a transmission coefficient curve, the
derivative is zero and the loop filter 604 provides the derivative
back as a direct current (DC) control output signal CO to the
signal generator 602. This closed loop operates to keep the signal
generator transmission output frequency at the peak absorption
frequency of the cell gas using lock-in differentiation based on
the RX signal received from the cell 508. The REF_CLK signal from
the signal generator 602 is the TX signal clock and can be provided
to other circuitry such as frequency dividers and other control
circuits requiring use of a clock.
[0039] The above discussion is meant to be illustrative of the
principles and various embodiments of the present invention.
Numerous variations and modifications will become apparent to those
skilled in the art once the above disclosure is fully appreciated.
It is intended that the following claims be interpreted to embrace
all such variations and modifications.
* * * * *